The present invention relates to improvements on immunity of a digital transmitter in transmission, and more particularly, to a method of detecting a carrier shift amount, a method of correcting the carrier shift amount, and a receiver employing these methods.
In recent years, an orthogonal frequency division multiplex (hereinafter, abbreviated as “OFDM”) modulation scheme, which is characterized by immunity to multipath fading and ghost, has gain the spotlight because of its suitability to applications in mobile digital transmission and ground-based digital television broadcasting.
The OFDM scheme, which is one of multi-carrier modulation schemes, digitally modulates n carriers (n ranges from several tens to several hundreds) which are orthogonal to one another.
A transmission signal from a transmitter in such a digital communication system can suffer from shifts in frequency on a transmission path due to influences such as fading. When a frequency-shifted signal is received, the signal must be corrected for the frequency shift before it is demodulated.
FIG. 1 illustrates a prior art example for correcting a received signal for a frequency shift. A signal received at a high frequency reception unit (not shown) of a receiver is frequency converted to an IF signal in the high frequency reception unit. The IF signal from the high frequency reception unit is again frequency converted to a baseband signal in a frequency converter 1. The resulting baseband signal, after converted from the received signal, is analog-to-digital converted in an AD (analog-to-digital) converter 3 (hereinafter called the “A/D conversion”). The digital signal resulting from the A/D conversion is orthogonally demodulated to an I-axis and Q-axis signal in an orthogonal demodulator 4. The Q-axis signal resulting from the orthogonal demodulation is applied to an error detector 8 to control a control voltage of a frequency control VCO (voltage controlled oscillator) 2 such that the value of the Q-axis signal reduces to zero. With this control, the frequency shift exerted on the signal in the midway of a transmission path is removed from the received signal. Such techniques are disclosed, for example, in JP-A-11-4209.